Infusion is the process of extracting chemical compounds or flavors from plant material in a solvent such as water, oil or alcohol, by allowing the material to remain suspended in the solvent over time (a process often called steeping). An infusion is also the name for the resultant liquid. The process of infusion is distinct from decoction, which involves boiling the plant material, or percolation, in which the water passes through the material (as in a coffeemaker).

An infusion is a very simple chemical process used with botanicals that are volatile and dissolve readily, or release their active ingredients easily, in water, oil or alcohol. The botanicals are typically dried herbs, flowers or berries. The liquid is typically boiled (or brought to another appropriate temperature) and then poured over the herb, which is then allowed to steep in the liquid for a period of time. The liquid may then be strained or the herbs otherwise removed from the liquid. Unless the infusion is to be consumed immediately, it may then be bottled and refrigerated for future use.

The amount of time the herbs are left in the liquid depends on the purpose for which the infusion is being prepared. Usually steeping for not more than 15 to 30 minutes, or until the mix cools, will create a beverage with optimal flavor. Steeping for a longer time typically results in a somewhat bitter-tasting infusion. Four (4) hours, however, is a more appropriate length of time for achieving herbal potency if health benefits are the priority. Quantities of the herb and liquid used will vary according to the herb or how strong the infusion is required to be. A common proportion used is 28 g (one ounce) of herb to 0.5 L (one pint) of liquid.

There have been several accessories and techniques for removing the steeped or left over products that were used to infuse liquids such as water, oil, or alcohol. The use of a metal steeper, which looks like a metal clamp. Tea infusers work as strainers and assist in removal of used herbs,leaves, etc., from over steeping or leaving residues. French presses are commonly used to infuse water with various teas and coffee. Lastly, and most commonly used, the tea bag. Tea bags today are made with filter paper and filled with various tea flavors.

A common example of an infusion is tea; most varieties of tea call for steeping the leaves in hot water, although some variants (e.g. Moroccan mint tea) call for decoction instead. Many herbal teas are prepared by infusion, as well; lemon, chamomile, senna, apple, ginger, rooibos, and a great many other plants are used individually or in combination. Herbal infusions in water and oil are both commonly used as herbal remedies. Coffee can also be made through infusion (as in a French press), but is more often made through percolation.

Plants with desirable flavors may be steeped in an edible oil or vinegar for an extended period; the infused oil or vinegar is often sold still containing the plant, and is then used as flavoring. Chiles, lemon, garlic, and many other plants may be used. There can be ambiguity in the labeling of these oils: for example, what is described as sesame oil may be oil extracted from sesameseeds, or another vegetable oil infused with sesame.